In music, the subtonic is the degree of a musical scale which is a whole step below the tonic note. In a major key, it is a lowered, or flattened, seventh scale degree (♭
The subtonic can be contrasted with the leading note, which is a half step below the tonic. The distinction between leading note and subtonic has been made by theorists since at least the second quarter of the 20th century. Before that, the term subtonic often referred to the leading tone triad, for example.
The word subtonic is also used as an English translation of subtonium, the Latin term used in Gregorian chant theory for the similar usage of a tone one whole step below the mode final in the Dorian, Phrygian, and Mixolydian modes.
The triad built on the subtonic note is called the subtonic chord. In Roman numeral analysis, the subtonic chord is symbolized by the Roman numeral "♭VII" in a major key. In a minor key, it is often written as "VII", the flat symbol being often omitted by some theorists because the subtonic note appears in the natural minor scale. The flat symbol is used for the major scale because the subtonic is a non-diatonic note.
Theorists Stefan Kostka and Dorothy Payne describe the subtonic chord (VII) as "sounding like the V in the key of the relative major—that is, a V of ♭III." Allen Forte writes that "[w]hile VII in relation to C minor (I) becomes V in relation to III (E♭ major).... As a major triad on an unaltered or natural scale degree 7 in minor the VII functions as a secondary dominant triad in relation to the mediant." In the minor mode, the subtonic chord may also appear as a major minor seventh chord (i.e. dominant seventh chord), ♭VII7.
In jazz, the flattened seventh is also used as a substitute for the dominant, V, especially in the backdoor cadence, ii–♭VII7–I, where the subtonic is substituted for the dominant seventh. In this case, ♭VII functions as a pivot chord borrowed from the parallel minor (its dominant seventh). The chords V7 and ♭VII7 have two common tones: in C major, these chords are G–B–D–F and B♭–D–F–A♭.
However, while "the leading-tone/tonic relationship is axiomatic to the definition of common practice tonality", especially cadences and modulations, in popular music and rock a diatonic scalic leading tone (i.e., ♮